1. Field of the Invention
The present invention relates in general to a conductive adhesive, and in particular to an anisotropic conductive adhesive. Still more particularly, the present invention provides a thermoplastic conductive adhesive that becomes uniaxially conductive upon the application of heat and a force.
2. Description of the Prior Art
In attaching electronic components to various surfaces, such as those on printed circuit boards or on other electronic components, it is desirable to use a composition or substance that is able to form a mechanical connection between the electronic component and the printed circuit board to hold the component in place. Moreover, the substance should be conductive in order to make an electrical contact between the component and the printed circuit board.
Presently, solder and conductive adhesives are some of the compositions utilized to form a mechanical and electrical connection between an electronic component and a printed circuit board. Solder has been used for many years to attach electronic components to printed circuit boards, but solder has a drawback since it contains toxic materials, i.e., lead. Moreover, as the leads extending from electronic components and the holes and contacts in which the leads are placed into or onto circuit boards become finer and finer and closer together, it becomes necessary to place precise amounts of the solder very accurately to avoid bridging or short circuiting between leads of an electronic component or leads between two or more electronic components.
The problem of bridging also occurred with conductive adhesives until anisotropic adhesives having the property of being uniaxially conductive were developed. The term "anisotropic" means exhibiting properties with different values when measured along axes in different directions. The term "uniaxially conductive" means e.g., conductive in a direction along a perpendicular axis intersecting two parallel planes generally normal to the perpendicular axis.
Conductive adhesive technology currently utilizes point contact between dispersed metallic particles to form a conductive path. In such a contact system, the mechanical integrity of the connection arises solely from the adhesive properties and the intrinsic strength of the polymeric matrix.
Increasing environmental concerns have led to a search for non-toxic alternatives. Many of these alternatives, such as gold or silver, are expensive and still are not very reliable. Currently, the anisotropic conductive adhesives that are available contain either gold or silver spheres dispersed in a polymer matrix or polymer spheres coated with silver or nickel. The spheres are forced into contact under an applied load which allows electronic percolation in localized regions. Such a system can experience problems such as electrical failure due to polymer expansion causing the spheres to lose contact with each other. In addition, these systems may also have problems with metal oxidation, causing high resistance paths at the interface.
Therefore, it would be desirable to have a method and apparatus for forming electrical and mechanical connections between electronic components that is non-toxic and low-cost.